Scheer Solar Economy Renewable Energy for a Sustainable Global Future (Earthscan, 2005)

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The potential for natural and technological solar energy storage

The lack of an adequate mechanism for storing electricity is the greatest handicap faced by PV and wind power. The current solution is to use the grid as a form of proxy storage: renewable energy is fed into the grid; when there is no wind or sun, conventionally generated electricity is supplied from the grid.

Operators of conventional power stations argue against this on the basis that the unreliability of wind and sun means that total operating time over the year is so low that economic production from renewable sources is not possible, even with a large number of PV and wind power installations feeding power into the grid. Peak operating time for well-situated windfarms is around 2000 hours in a year of 8760 hours, albeit this is increasing as the technology improves; for PV, peak operating time is less than 1000 hours. In reality, installations run for longer than this – around 4000 hours in the case of wind – but at reduced output. This puts into perspective the electricity industry’s plea that, while they have to have capacity in place to cover for wind and PV downtime, it languishes unused when electricity is being generated from these sources. For as long as wind and PV output is a negligible proportion of total production, this is a flimsy argument. Electricity producers have so much underused capacity.

Nevertheless, ‘capacity effects’ do come into play when a large proportion of energy is generated from wind and PV. In this case, there would be a need for reserve capacity that would indeed be underused during wind and PV operating hours. Targets for increasing the share of electricity from wind and PV are thus sowing the seeds for future capacity conflicts between operators of PV and wind installations and the conventional electricity suppliers. Consequently, the industry is demanding that feed-in legislation for renewable energy be dropped, or upper limits imposed on the ‘forced purchase’ of renewable energy.

One possibility for postponing the capacity conflict between PV and wind and conventional power stations would be to expand the scope of the grid. There is, however, an inter-

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nal contradiction in the idea that the economic viability of decentralized plant should depend on a large energy grid, of all things. It also makes electricity from PV and wind unnecessarily expensive, as transmission and distribution make up around 60–80 per cent of electricity costs. The only reason that this is not more obvious is that electricity bills make no mention of it, and the electricity industry does not publish any precise statistics on the subject.

If the economic dynamism latent in PV and wind power is ever to blossom, new answers must be found for two key questions:

1How can the need for expensive reserve capacity be avoided in future?

2How can the unique economic advantage of renewable energy sources, in that their exploitation does not require long supply chains or an overarching energy supply system, be made to count for wind and PV power in practice? To put it more tendentiously: how can they be made independent of anonymous reserve capacity and even independent of high-performance energy grids?

Hybrid systems: electricity supply without anonymous reserves

The first point to note is that reserve power stations lose their significance if the grid is fed from numerous small local power stations. Unlike large power stations, it does not matter if a small power station drops out. A system built of large numbers of small plants is inherently multiply redundant. Secondly, continuity of supply is not an argument that sensibly applies to a market economy in which the value of a good is determined by the interaction of supply and demand. Thirdly, the correct response to the argument that you can’t dry your laundry if the wind doesn’t blow is that you can dry it when it does blow.

One suggestion for solving the problem would be so-called hybrid systems that can generate electricity from two different sources – and above all harnessing all known and as-yet

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unknown technologies for storing electricity once generated. With hydropower from reservoirs, it is of course possible to meet all electricity demand from one source, since it is possible to react immediately to changes in demand by swiftly controlling the water flow to turn turbines on or off. Where there are enough suitable sites, as in Norway, the entire national demand can easily be met from hydropower stations. Many smaller regions could also make themselves independent of external electricity suppliers. Yet they choose rather to sell their hydroelectricity far and wide. Its suitability for meeting peak demand commands a high price on the energy market. Any further source of electricity can supply an electricity market’s needs if combined with sufficient hydroelectric capacity, be it fossil fuel or renewable. France has a hybrid system that combines hydropower with nuclear energy in the ratio of 1:3. A strategy that works at national level can quite clearly meet all electricity needs on the small scale – without nuclear power and without fossil fuels, but also without the universal availability of hydrolectricity from reservoirs.

Another possible hybrid would be wind power in combination with a biomass plant. Whenever the wind dropped, but demand for electricity remained high, a biogas-, vegetable oilor gasified biomass-burning generator linked to the windfarm would automatically start up, and stop again as soon as the windfarm took the strain again. A plant like this could supply the grid according to need, or could guarantee a fully autonomous power supply. The argument that a fossil-fuel- free energy supply cannot be achieved without recourse to geographically limited reserves of hydroelectricity quite literally does not hold water. There are other reasons why such a hybrid supply is not the only and not even the optimum strategy for renewable energy. A biomass-fired generator serving as reserve capacity for a wind farm would not be fully utilized, since it could be providing power round the clock. Furthermore, the primary energy could be used most efficiently if the waste heat were captured and put to use as well. The traditional seasonal differences in energy usage, however, make it scarcely possible to guarantee custom for both heat and electricity if the generator is run at constant capacity. All these factors make the question of direct storage of electricity a pressing issue.

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Technologies for storing solar energy

In 1999, the Society for Innovative Energy Generation and Storage (EUS) commissioned an electricity storage plant in Bocholt, Germany. This plant stores electricity from four wind turbines with a total output of 3.5 MW in a 1.6 MW battery, and feeds it into the grid whenever demand is greatest. Due to the greater returns enjoyed by this plant, it is expected to have paid for itself in around six years, despite being the first installation of its kind.8 This and other storage technologies open the door to far-reaching economic opportunities for renewable energy to blossom in the electricity market. Once it becomes possible to store electricity, all the arguments against supplying electricity from renewable sources relating to capacity and productivity lose their sway, as do previous arguments for the existence of a national grid. Low-cost storage technologies enable a qualitative leap to exploiting renewable energy across the entire electricity supply system. Ultimately, decentralization of energy supplies will become unstoppable.

The spectrum of potentially useful storage technologies ranges from electrochemical, electrostatic and electromechanical to thermal and chemical media. Most widespread so far has been electrochemical storage, in the form of batteries. There industry let it rest for a long time, in the absence of perceptible demand for other, better options. There were many reasons for this status quo. Though the idea of electric vehicles had been around for a century, so few were built that there was no pressure on industry to develop lighter, more powerful batteries. Regional monopolies also meant that electricity companies had no incentive to devise new storage media to cover peak demand, for example: instead, they preferred to rely on dams, reservoirs and pumps. Only for submarines were more powerful lead-acid batteries developed. These subsequently came to dominate the industry.

The most effective pressure to develop new storage technologies in the past two decades has come from the environmental movement, which demanded less polluting batteries, and from the electronics and space industries. The latter’s need for tiny mains-independent devices with an

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extremely low power rating led to the development of electrostatic supercapacitors, which are also vital for the development of solar-powered stand-alone and always-on devices as outlined earlier. Recently, the car industry has found a need for powerful batteries to optimize the technology for electric cars, for which the greatest spur so far has been California’s environmental legislation. This requires 10 per cent of all cars sold in the state to be zero-rated for emissions by 2003. In the mid1990s, the US government also initiated a $260 million research programme in battery technology, in which all US car manufacturers are participating.9

Electricity storage has up to now played only a secondary role in publicly funded research into solar technology, despite its central importance for renewable energy supplies. Hence it is important to be aware of the whole spectrum of options available. Given the decades-long neglect of electricity storage technology, none of these options can have had time to mature; nevertheless, the range of possibilities is broader than many people realize, and it is possible in some cases to draw on tried and tested technology, which, in combination with renewable energy, can now be put to new and undreamed-of uses.

Electrochemical accumulators

In electrochemical batteries, power flows in through one electrode and out through a second. In the process, the energy content of the chemical substance between the electrodes is increased. The process is reversible, and can be repeated thousands of times. The commonest form of electrochemical accumulator is the flooded or hermetically sealed lead-acid battery, which is now largely a mature technology. They are cost-effective and highly efficient, but have a low energy density, and their disposal causes considerable problems. Better energy densities, albeit with lower efficiency, are offered by nickel-metal-hydride batteries; but here too there are problems with disposal.

One new type is the redox battery (short for ‘reduction and oxidation electrolyte circulation’), which has a viscous fluid electrolyte. Once drained of charge, the fluid is pumped out at

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a garage, and replaced with charged fluid. Changing the electrolyte fluid saves the user the hours required to recharge the battery, thus making them particularly suitable for use in electric cars. Efforts to drastically reduce battery weight are also directed at electric cars: lighter batteries would extend their range.

More promising for the issue in question, however, are lithium ion or lithium polymer batteries, which take the form of a thin film. The technology is still very new, and the costs correspondingly high. But efficiency and energy density are high, weight is negligible, and the batteries are good for innumerable cycles, environmentally sound and require virtually no maintenance. Lithium ion batteries also do not need special chargers. They make particular sense for PV because the batteries can be built into the panels, thereby integrating generation and storage in one unit. Building roofs and façades would also be suitable storage surfaces.

Electrostatic storage

Supercapacitors come into this category. Electricity is stored without loss in a solid electrolyte, and no chemical change takes place. Supercapacitors are light and can be extremely small. Though still immature, the technology combines high energy density and efficiency with low environmental impact. Their working lifetime is greater than for all other battery types, stretching into the millions of charge/discharge cycles. The cost, though, is still high, and current models are not very powerful, having been developed for low-power electronics. The first supercapacitors could store no more than a few ampereseconds; this has since been increased to an ampere-hour. Currently to be found in wristwatches, mini-radios and measuring instruments, supercapacitors are vital to the technical development of stand-alone and stand-by devices. They offer considerable additional scope for reducing energy use in all electrical devices, thereby smoothing the way for cost-effective growth in PV.

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Fly-wheels

Fly-wheels are a form of electromechanical storage. The flywheel is a rotating cylindrical body, and the amount of energy stored grows in proportion to the mass of the fly-wheel and the square of its speed of rotation. The stored energy can be use to drive a motor or to smooth out short-term fluctuations in the supply or flow of energy. Fly-wheel technology has a variety of applications from cassette recorders to motorbikes, but also including motor vehicles and motor generators. Years of neglect have left the technology underdeveloped, but energy densities are high and there is no waste disposal problem.

Researchers are currently experimenting with magnetic fields as a means of achieving higher speeds and reducing the loss of stored energy due to friction caused by the weight of the spinning mass. An electrical motor and braking system is used to control the speed of the fly-wheel, which currently reaches around 120,000 revolutions per minute. Fly-wheels are easy to manage and can be scaled down, which makes them suitable for local autonomous supplies, to bridge gaps in supply from wind or PV.

Compressed air

Compressed air is a tried and tested technology that can quickly be deployed for electricity storage. Factories used to drive their machinery with it; now compressed air is used to enhance the performance of Formula 1 and aircraft engines. Compressed air is another form of electromechanical storage. Electrical energy drives air compressors which pump air into high-pressure tanks. The stored air is then used to drive generators or motors as required. Compressed-air tanks are well understood, the costs are relatively low, and energy density average.

The first public demonstration of a car that runs solely on compressed air took place in 1999, the result of a collaboration between former Formula 1 motor engineer Guy Negre and the Luxembourg firm MDI. The car needs 20 kWh of electricity to fill a 300 litre tank, which gives it an urban range of 200 km (125 miles). The car’s top speed is 110 km/h (70 mph).

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With electricity costing €0.10 ($0.09) per kWh, it is possible to travel 200 km for €2 ($1.80). The motor draws in normal atmospheric air, and this is fed along with the compressed air into a cylindrical chamber. The heat expansion of both air inputs drives pistons in a neighbouring cylinder, which in turn drive the car. The entire two-cylinder motor weighs only 35 kg. The relatively low weight of the compressed air tank is a considerable advantage over today ’s electric vehicles, whose range is limited by their very heavy batteries. Using external compressors, for example at a compressed air station, ‘refuelling’ takes only three minutes; with the on-board compressor plugged into the mains, it takes four hours. Servicing is supposed to be necessary only every 100,000 km; the only emission is cold air. The efficiency is 85 per cent of the electricity needed to run the compressor.

Compressed air does not just make sense for cars. Fixed plant for energy storage in buildings, for example, is even easier to manage. A 15,000 litre tank, comparable to a moderately large domestic oil tank, could store 1000 kWh that could be converted into electricity with the help of a motor. That would be sufficient for an autonomous electricity supply, something that could previously only be achieved with a cellarful of batteries, each with an operational lifespan of only up to 2000 charge/discharge cycles. The number of compress/decompress cycles achievable with a compressed air tank is in principle unlimited, which means that small autonomous units can get by without a grid connection. If building-mounted PV panels are supplemented with a small wind turbine, with a capacity of perhaps 1 kW, or a small Stirling engine, so that the pressure tank can be topped up even when the sun does not shine, then the tank need be no bigger than an average domestic oil tank. It would also be possible to turn wind turbines into compressed air stations, or to combine wind turbines with large compressed air motors to provide a truly 24-hour electricity supply.

Electrodynamic storage

The medium in this case is an electromagnet, like the spark plugs in a car engine. Current is piped into a superconducting

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coil to create a magnetic field, from which current is subsequently drawn. However, the technology is still very much in the experimental phase. The superconducting coils have to be cooled to 170 degrees absolute, and the relationship between input and output for the stored electricity is still unclear. Moreover, the system is highly complex and heavy.

Solar-powered electrolysis

The power storage option that offers the widest variety of applications is electrolytic extraction of hydrogen, by which electrical energy is converted into chemical energy. Electrolysis is a long-established process; the primary focus of development work is improved efficiency. The electrolysis equipment consists of a cathode (the negative electrode) and an anode with a water-based electrolyte in between. Electrons are forced out of the cathode into the electrolyte, and the resulting chemical reaction releases hydrogen. The anode (positive electrode) sucks electrons out, causing a second reaction which releases oxygen. It is vital to keep the hydrogen and oxygen gases separate. Hydrogen has a high energy density, and therefore requires little storage space. Its extreme versatility makes it the ideal fuel.10

What matters is how the hydrogen is produced. If the electricity used comes from nuclear or fossil-fuel power plants, the result is environmental self-deception. Although the fuel is clean – the only emission being the water vapour produced by combusting hydrogen in oxygen – no substitution of nuclear or fossil fuel energy takes place.

The overwhelming majority of schemes for hydrogen production from renewable energy envisage using large power stations – large dams or solar thermal plants in arid and semiarid regions – to mass-produce hydrogen for subsequent delivery to the end-user. The other option for solar-powered hydrogen electrolysis would be a locally based approach, using electricity from PV or wind. Arguments in favour of this route relate to the opportunities for autonomous fuel production or for storing self-generated electricity rather than feeding it into the grid.

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Thermal storage

The question of how better storage options can ensure or further develop energy self-sufficiency is also germane to solar heating. It goes without saying that every solar collector needs a hot water storage tank. The pattern thus far has been, however, that solar heating provides only one part of the necessary heat, with additional heating needs being met from conventional sources. The natural next step is therefore to seek complete independence from fossil fuel top-up supplies. From a technological point of view, this does not present a problem: it just takes a larger collector area, greater storage capacity and less need for heat – for example, with better insulation, heat exchangers, heat reclamation and optimal passive solar gain for the building as a whole. This line of attack has led to many successful zero-emission housing projects, recently even including some by mainstream developers.

But enlarged collector area and greater storage capacity is not the only answer. Another option is the solar magnesium hybrid storage system developed by Hans and Jürgen Kleinwächter in cooperation with the Max Planck Institutes in Müllheim and the Ruhr. The system works by using mirrors to concentrate heat on the storage unit, where the heat energy separates hydrogen from magnesium. The hydrogen can then be used as a heat-transport medium to drive a Stirling engine producing electricity and hot water for the heating system. Once the hydrogen has recombined with the magnesium, the cycle can begin again.11

Thermal plants could also do with making better use of seasonal variations in temperature. This is particularly relevant in the case of CHP, or cogeneration, which currently still largely runs on fossil energy. The problem with CHP plants is that as the cogeneration sector grows, it becomes increasingly difficult to find customers for the spare heat. It would make more sense to use the heat to drive Stirling engines producing additional electricity. This electricity can then either be consumed immediately, or stored for later use, as described above. Stirling engines are thermal power plants which do not need a fixed fuel input, being able to convert any external heat source into mechanical or electrical energy.12